Zone	Fahrenheit  Celsius	
1	Under -50 F     Under -45.6 C
2a	-50 to -45 F    -42.8 to -45.5 C
2b	-45 to -40 F    -40.0 to -42.7 C
3a	-40 to -35 F    -37.3 to -39.9 C
3b	-35 to -30 F    -34.5 to -37.2 C
4a	-30 to -25 F    -31.7 to -34.4 C
4b	-25 to -20 F    -28.9 to -31.6 C
5a	-20 to -15 F    -26.2 to -28.8 C
5b	-15 to -10 F    -23.4 to -26.1 C
6a	-10 to -5 F      -20.6 to -23.3 C
6b	-5 to 0 F          -17.8 to -20.5 C
7a	  0 to 5 F          -15.0 to -17.7 C
7b	  5 to 10 F        -12.3 to -14.9 C
8a	 10 to 15 F       -9.5 to -12.2 C
8b	 15 to 20 F       -6.7 to -9.4 C
9a	 20 to 25 F       -3.9 to -6.6 C
9b	 25 to 30 F       -1.2 to -3.8 C
10a	 30 to 35 F         1.6 to -1.1 C
10b	 35 to 40 F         4.4 to 1.7 C
11	 over 40 F          over 4.5 C